In the prototype version of the game, Raidou is given an end card sequence, hinting that he would have been unlockable in the final version.

"After all, this is all it is."
A long time had passed since Raidou was expelled from the Mugen Tenshinryuu village. He had only hungered for power ever since. He fought all and every fighter and claimed his technique his own.
Even as he received the invitation to participate in the tournament, he felt this might sate his hunger for power.
However, it seems all was in vain.
Raidou called for thunder and disappeared into the mist...
Contributed by DrakeVagabond